FFM-12192 Remove size checks from metrics maps & lock both map writes

[erdirowlands]

What

• Removes size checks for evaluation and target metrics maps, as these are not needed and complicate concurrency.
• Also synchronises the increment/put operation on both maps. In 1.4.2, decision was taken to only synchronize the clone and clear operations, to stop multiple threads from doing the same thing. However, for extra safety, we now synchronise the writes.
• Declares and assigns cloned maps in one atomic step.

Testing

Testgrid Manual tests